Silicon on insulator (Silicon-On-Insulator, SOI) technology is an all-dielectric isolation technology, namely, a buried oxide layer lies between the top silicon film and the substrate for isolating the film of active devices from the substrate. Devices build on the SOI technology are known as SOI devices. As shown, FIG. 1 illustrates a diagram of a cross-sectional view of the structure of a typical SOI CMOS structure in the prior art. Excellent isolation can be achieved owing to the existence of a buried oxide layer; the volume of the active region of the SOI device is rather small, so the SOI device has a lower leakage current than body silicon does and has no latch-up effect as well; thus, SOI devices show incomparable advantages over other devices in the term of anti-radiation performance. However, it is exactly because of the existence of the buried oxide layer, there are a lot of hole traps inside the SOI device. Consequently, when the SOI device continues to work in an ionizing radiation environment, ionizing radiation excites electron-hole pairs within the buried oxide layer, then electrons will soon migrate out of the buried oxide layer, but the holes will be captured by the hole traps, which then become fixed spatial positive charges and result in accumulation of positive charges. These fixed spatial positive charges mainly aggregate near the interface of the top silicon film and the buried oxide layer. When the positive charges accumulated inside the buried oxide layer reach a certain extent, the back gate interface of SOI N-channel transistors will turn into anti-configuration, which consequently results in increase of the leakage current in the device, drift of the electrical characteristics parameters and eventually failure of the device. Therefore, researches are currently focusing on how to improve the total dose anti-radiation performance of SOT devices.
In the prior art, following two ways are adopted to improve anti-radiation performance of SOI structures, so as to improve the total dosage anti-radiation performance of SOI devices.
The first way is to improve back gate threshold voltage by implanting Boron cations into the silicon substrates of SOI structures so as to improve anti-radiation performance of SOI devices.
The second way is to introduce to the buried oxide layer deep electron traps or recombination centres so as to prevent electrons generated by the radiation from migrating out of the buried oxide layer, which thus is able to maintain electrical neutrality in the buried oxide layer and thereby improving anti-radiation performance of the buried oxide layer. Ions introduced into the buried oxide layer should be able to combine easily with electrons generated by the radiation, and the combination cannot be separated easily; nor will the ions be allocated afresh in the buried oxide layer. Usually, the common practice is to implant ions (e.g., Si, N, Al or the like) into the buried oxide layer, and to introduce into the buried oxide layer deep electron traps or recombination centres. Take implantation of Si into the buried oxide layer as an example. Implanted Si forms electron traps within the buried oxide layer; when these electron traps are filled, they will compensate trapped positive charges, thereby reducing net positive charges in the buried oxide layer. Take N implantation into the buried oxide layer as an example. In respect of a typical SOI device, given the existence of hole traps of high concentration within the buried oxide layer, the holes in the electron-hole pairs in the whole buried oxide layer generated by radiation are almost captured in situ, while the electrons are captured by electron traps at shallow energy level and then thermally evaporate quickly. Electrons that are thermally excited are subsequently swept out of the buried oxide layer by the electrical field. In this case, the buried oxide layer carries positive charges in the macro sense, which would adversely affect the normal use and performance of the SOI devices. Implantation of N ions will produce a large number of electron traps within the buried oxide layer and, after annealing process, N ions combine with Si thereby forming Si—N bonds with fairly strong bond energy, which thence substitute for some weak bonds. The substitution for weak bonds is able to reduce electron-hole pairs in the buried oxide layer generated by the ionizing radiation, thereby improving anti-radiation performance of SOI devices. Besides, during the process of ionizing radiation, the electrons captured by the electron traps not only can function as recombination centres for attracting holes generated by the radiation, but also can compensate the holes that have been captured by hole traps; this helps to achieve electrical neutrality in the macro sense. Additionally, these electron traps further affect electrical field in the buried oxide layer, which is favourable for recombination of electron-hole pairs generated by the radiation.
However, the above two ways have some shortcomings. Specifically, the first way is reduced to such a shortcoming as limited capacity to improve anti-radiation performance of SOI structures. The shortcoming of the second method is: it is inevitable to cause some implanting damage to the top silicon film during the process of implanting ions into the buried oxide layer of the SOI structure. Although the damage can be substantially eliminated through annealing performed after the implantation, some defect left after the annealing still casts influence on the performance of the SOI device. Furthermore, although ion implantation can effectively improve anti-radiation performance of the buried oxide layer, it may affect the internal microstructure of the buried oxide layer, which is represented by changes to the electrical characteristic of the buried oxide layer in the macro sense. But the change as such may in turn affect improvement to the anti-radiation performance of the SOI structure that may be achieved through ion implantation. For example, N ion implantation of high dosage will affect the structure of the buried oxide layer, which consequently causes relaxation to the originally dense SiO2 atom network, then Si—O—Si bond angle becomes larger; this consequently causes more holes to be trapped in the buried oxide layer, and is adverse to the anti-radiation performance of the SOI device. Meanwhile, N implantation will cause decrease in back gate threshold voltage of the NMOSFET but increase in back gate threshold voltage of PMOSFET.
Accordingly, there is an urgent need to put forward a method capable of improving anti-radiation performance of an SOI structure but being free from aforementioned problems.